The present invention relates to a method of making highly sinterable lanthanum chromite powder, which can be used in making a connection layer on an electrode of an electrochemical cell.
High temperature electrochemical cells are taught in U.S. Pat. No. 4,490,444 (Isenberg). In these types of cells, typified by fuel cells, a porous support tube of calcia stabilized zirconia, has an air electrode cathode deposited on it. The air electrode may be made of, for example, doped oxides of the perovskite family, such as lanthanum manganite. Surrounding the major portion of the outer periphery of the air electrode is a layer of gas-tight solid electrolyte, usually yttria stabilized zirconia. A selected radial segment of the air electrode is covered by an interconnection material. The interconnection material may be made of a doped lanthanum chromite film. The generally used dopant is Mg, although Ca and Sr have also been suggested.
Both the electrolyte and interconnect material are applied on top of the air electrode by a modified electrochemical vapor deposition process, at temperatures of up to 1,450.degree. C., with the suggested use of vaporized halides of zirconium and yttrium for the electrolyte, and vaporized halides of lanthanum, chromium, magnesium, calcium or strontium for the interconnection material, as taught in U.S. Pat. No. 4,609,562 (Isenberg et al.).
U.S. Pat. No. 4,631,238 (Ruka), in an attempt to solve potential interconnection thermal expansion mismatch problems between the interconnect, electrolyte, electrode, and support materials, taught cobalt doped lanthanum chromite, preferably also doped with magnesium, for example La.sub..97 Mg.sub..03 Co.sub..04 Cr.sub..96 O.sub.3, as a vapor deposited interconnection material, using chloride vapors of lanthanum, chromium, magnesium, and cobalt.
It has been found, however, that there are certain thermodynamic and kinetic limitations in doping the interconnection from a vapor phase by an electrochemical vapor deposition process at 1,300.degree. C. to 1,450.degree. C. The vapor pressures of the calcium chloride, strontium chloride, cobalt chloride, and barium chloride are low at vapor deposition temperatures, and so, are not easily transported to the reaction zone at the surface of the air electrode.
Thus, magnesium is the primary dopant used for the interconnection material. However, magnesium doped lanthanum chromite, for example La.sub..97 Mg.sub..03 CrO.sub.3, has a 12% to 14% thermal expansion mismatch with the air electrode and electrolyte material. Additionally, formation of an interconnection coating solely by electrochemical vapor deposition can lead to interconnection thickness variations along the cell length. Then, thin portions would be subject to possible leakage, and thick portions would be subject to increased thermal expansion stresses. Finally, electrochemical vapor deposition of these materials as thin, dense, leak-proof films is an expensive and complicated procedure.
U.S. Pat. No. 4,861,345 (Bowker et al.), in a completely different approach, taught depositing particles of LaCrO.sub.3, doped with Sr, Mg, Ca, Ba or Co and coated with calcium oxide or chromium oxide, on an air electrode, and then sintering in air at 1,400.degree. C. Here, the metal of the surface deposit was absorbed into the LaCrO.sub.3 structure. This process completely eliminated vapor deposition steps and the skeletal support structure.
However, lanthanum chromite compounds are known to be difficult to sinter in air without the application of pressure, as taught by Anderson, "Fabrication And Property Control of LaCrO.sub.3 Based Oxides", in Materials Science Research, Vol. 11, Plenum Press (1977), pp. 469-477. Anderson concluded that sintering Cr-containing oxides to a high density required controlling the oxygen activity in the furnace by flowing CO/CO.sub.2, H.sub.2 /CO.sub.2, or H.sub.2 /H.sub.2 O gas mixtures through the furnace during sintering, in order to control Cr.sup.3+ ion volatilization. Some substitution of Al for Cr was also suggested.
Group et al., J. Amer. Ceram. Soc., Vol. 59, No. 9-10, (1976), pp. 449-450, had also recognized the difficulty in sintering LaCrO.sub.3 by normal techniques, primarily due to volatilization of Cr oxide compounds in oxidizing atmospheres. They prepared compositions containing up to 20 mole % Sr by dissolving nitrates of the constituent La, Sr, and Cr cations in a solution of citric acid and ethylene glycol, followed by evaporation at 135.degree. C., to provide a glass-like resin, which was then calcined at 800.degree. C., to provide a La.sub.1-x Sr.sub.x CrO.sub.3 material. Powder samples of this material, with distilled water as binder, were uniaxially pressed, at 2,115 kg./cm..sup.2 (20.685 MPa), to provide discs of 55% to 60% theoretical density, which were then sintered in the temperature range of from 1,600.degree. C. to 1,700.degree. C. for 1 hour, at oxygen activities of from 10.sup.-12 to 10.sup.-11 atm., to provide compacts having maximum densities of 95%+.
Meadowcroft et al., Ceram. Bull., Vol. 58, No. 6, (1979), pp. 610-615, also recognized oxidation and vaporization problems with Sr or Ca doped LaCrO.sub.3 in air at over 1,600.degree. C. They mixed La.sub.2 O.sub.3 and Cr.sub.2 O.sub.3 with SrCO.sub.3, in appropriate amounts, and prefired the mixture in air at 1,400.degree. C. The reacted powder was first uniaxially, and then isostatically pressed, and fired at 1,500.degree. C. in air. The influence of substitutions on vaporization rate was studied for: EQU La.sub.1-x Sr.sub.x CrO.sub.3 (0&lt;x&lt;0.2); EQU La.sub.0.08 Sr.sub.0.2 Al.sub.0.5 Cr.sub.0.5 O.sub.3 ; EQU La.sub.0.8 Sr.sub.0.2 Al.sub.0.25 Cr.sub.0.75 O.sub.3 ; EQU La.sub.0.8 Mg.sub.0.2 CrO.sub.3 ; and EQU La.sub.0.8 Ca.sub.0.25 Cr.sub.0.75 O.sub.3.
The lowest vaporization rate was achieved for the calcium aluminum containing material.
Eror and Anderson, in "Polymeric Precursor Synthesis of Ceramic Materials", Mat. Res. Soc. Symp. Proc., Vol. 73, (1986), pp. 571-577, taught mixing the raw materials needed to make chromites, such as Cr.sub.2 O.sub.3, LaCrO.sub.3, MgCr.sub.2 O.sub.4, and the like, with anhydrous citric acid and ethylene glycol; heating the mixture to form a solution; evaporating the solution until an amorphous organic polymer formed; charring the solid at 400.degree. C. to provide a brittle mass; grinding and screening; and then calcining in air at 700.degree. C. to 800.degree. C., to provide a homogeneous single phase of precise cationic stoichiometry and particle size.
In the teachings of Eror and Anderson, there was no precipitation in the solution as it was evaporated to form a rigid polymeric state in the form of a glass, and all reactions were apparently conducted in air. Upon analysis, the final material appears to have low specific surface areas, in the order of 0.5 m.sup.2 /g to 4 m.sup.2 /g, which could provide difficulty in getting small particles. The authors note a disadvantage of their process is that powder agglomerates are hard, so that a dispersion of monosized particles might be difficult, and the process can take from 1 day to 3 days to complete. The grinding step in particular takes a long time, due to the hardness of the brittle char.
Another process for making lanthanum chromite powder for solid oxide fuel cell interconnections has been described by Chick et al., in "Synthesis Of Air-Sinterable Lanthanum Chromite Powders", Proceedings Of The First International Symposium On Solid Oxide Fuel Cells, Vol. 89-11, (1989), pp. 170-186. They recognized that sintering lanthanum chromites in air, even at temperatures above 1,700.degree. C. resulted in low densities and substantial open porosity. They also recognized that the powder should consist of dense, spherical, submicron sized particles in discrete rather than agglomerated form. They made such particles using a glycine/nitrate powder synthesis technique, where metal nitrates and glycerine were dissolved in water, and then the solution was boiled until it thickened and ignited, producing ash that contained the oxide product. While agglomerates were produced, they were soft, not hard, and could be broken down into discrete monosized particles with minimal effort. They also found that when the composition of glycine/nitrate-produced Sr-substituted lanthanum chromite powder was adjusted so that the calcined material contained 3 mole % to 5 mole % SrCrO.sub.4, densification in air was enhanced, probably due to the presence of a liquid phase at the sintering temperature. This process involved spontaneous ignition. Its use on a large commercial scale may involve expensive or complicated safety precautions or equipment.
None of the proposed solutions solve all the problems of thermal expansion mismatch, and, problems associated with doping calcium, strontium, cobalt, and barium into interconnections without electrochemical vapor deposition, or of providing a uniform, leak proof interconnection thickness in a simple, fast, economical, and 100% safe fashion. It is an object of this invention to solve such problems.